The growth phase of plants generally includes a vegetative growth phase and a reproductive growth phase. The transition from vegetative to reproductive growth is affected by various flowering signals. The flowering signals are affected by various factors, such as genetic factors such as genotype, and environmental factors such as photoperiod and light intensity, etc. (Dung et al., 1998; Yamamoto et al., 1998). The transition in the growth phase leads to various morphological changes of plant, which is interesting from a scientific viewpoint. Furthermore, due to the economic benefits gained by flowering regulation, many studies on flowering mechanisms have been carried out.
In particular, molecular genetic studies of Arabidopsis (hereafter, Arabidopsis) have shown the functions of the flowering regulatory genes and their interrelationships, elucidating the signaling pathway of flowering. In Arabidopsis, the flowering is affected by external signals, such as light, temperature, photoperiod, etc., and internal signals such as nutritive conditions, hormones, etc. The flowering pathway generally includes the photoperiod-dependent pathway, the vernalization-dependent pathway, the GA (gibberellin)-dependent pathway, and the endogenous pathway.
In rice, it has been reported that flowering-time is mainly controlled by the photoperiod-dependent pathway and the endogenous pathway (Yamamoto et al., 1998). Recently, several genes which are thought to be involved in the photoperiod-dependent pathway in rice have been isolated and identified, characterizing the photoperiod-dependent pathway to some degree. (Yano et al., 2001 Mouradov et al., 2002). Firstly, several specific gene loci which are involved in controlling photoperiod sensitivity, such as Se (Photoperiodic sensitivity)1 (Se1), Se3-Se7, and EE1-E3, have been identified (Poonyarit et al., 1989; Sano, 1992; Tsai, 1995; Yokoo et al., 1980; Yokoo and Okuno, 1993). (Furthermore, by quantitative trait loci (QTL) analyses using molecular level markers, several tens of gene loci involved in controlling heading date, such as Heading date 1 (Hd1), Heading date 6 (Hd6), Heading date 3a (Hd3a), etc., have been detected (Li et al., 1995; Lin et al., 1996, 1998; Maheswaran et al., 2000; Yano et al., 1997; Xiao et al., 1995). Among the above genes, Se5, Hd1, Hd6 and Hd3a are counterparts of LONG HYPOCOTYL 1 (HY1), CONSTANS (CO), CASEIN KINASE 2 (CK2) and FLOWERING LOCUS T (FT) of Arabidopsis, respectively, and they are expected to have biochemical functions similar to the counterparts in Arabidopsis. 
However, in some cases, the above genes of rice and Arabidopsis show different responses to photoperiod. CO of Arabidopsis is a gene which is involved in long-day (LD) promotion pathway to activate flowering. There are orthologs of CONSTANS in rice, and among them, Hd1 (Heading date1) gene regulates the flowering-time. Hd1 gene of rice, which encodes a protein containing a zinc finger domain and a nuclear localization signal, increases expression of Hd3a gene to activate flowering under short-day (SD) conditions, whereas it decreases expression of Hd3a gene to inhibit flowering under LD conditions (Izawa et al., 2002). In Arabidopsis, the CO gene, which is an ortholog of Hd1 of rice, increases expression of the FT gene (an ortholog of Hd3a of rice) to activate flowering under long-day conditions. However, the CO does not act as a flowering inhibitor under short-day conditions (Putterill et al., 1995). The molecular level understanding of such different photoperiod reactions depending on plant species is expected to provide a clue for understanding the differences between LD plants and SD plants.
Although flowering-relating genes such as the above have been discovered, they are few in number. Furthermore, rice-specific genes distinguished from Arabidopsis genes have been mostly unknown, and long-day specific flowering regulators and regulators for controlling the endogenous pathway which is different from the photoperiod pathway also have been mostly unknown. Considering that flowering-time is a key trait in determining cropping season and regional adaptability, and that significant agricultural profit can be obtained by controlling flowering-time, it is necessary to elucidate the flowering-time regulating pathway in rice and to find the genes involved in the pathway.
The present inventors identified useful flowering regulators in rice and investigated the characteristics thereof, to achieve the present invention. In the present invention, previously unknown flowering regulators have been, found, as well as the fact that their functions are specifically differentiated in rice.